1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus that obtains an image of a subject based on a nuclear magnetic resonance (NMR) signal emitted from the subject, and a control method thereof.
2. Description of the Related Art
To image a coronary artery based on the magnetic resonance image (MRI) method, a method of using a three-dimensional (3D) steady-state free precession (SSFP) sequence to perform imaging in a breath holding state or a voluntary breathing state is used. In case of whole heart MR coronary angiography (WH MRCA) where a course of a coronary artery of an entire heart is imaged in particular, holding a breath may lead to an insufficient spatial resolution in some cases.
As a countermeasure, there is used an realtime motion correction (RMC) method of detecting a position of, e.g., a diaphragm based on an nuclear magnetic resonance (NMR) signal under voluntary breathing to perform imaging while monitoring a respiratory level and changing an imaging position in accordance with this respiratory level.
However, a variable amount of the position that enables accurate imaging is restricted more or less, there is adopted a method of providing a fixed threshold value with respect to a movement range obtained by respiration and pausing collection of the NMR signal for imaging when the movement is large beyond this threshold value. That is, for example, a position of the diaphragm in a body axis direction can be detected from a signal (which will be referred to as a monitor signal) obtained by subjecting an NMR signal collected in relation to such a region R as shown in FIG. 1 to one-dimensional Fourier transformation. Since the position of the diaphragm in the body axis direction cyclically moves up and down in accordance with respiration, plotting the cyclically detected positions of the diaphragm in time-series enables obtaining such a monitor signal as depicted in FIG. 2 that is synchronized with a respiratory motion. When a peak of this monitor signal is out of an allowable range between an upper threshold value USL and a lower threshold value LSL as shown in FIG. 2, imaging is not performed or collected data is not used. When the monitor signal falls within the allowable range, data collection is carried out. Further, imaging is effected while changing an imaging position in accordance with the respiratory motion.
Performing the operation in this manner enables excellently obtaining a 3D image having a resolution that is high even under voluntary breathing.
However, when the respiratory level is not fixed and gradually lowered or gradually increased and a portion of the signal obtained by subjecting the NMR signal to one-dimensional Fourier transformation that corresponds to a position of the diaphragm deviates from the allowable range as shown in, e.g., FIG. 3, an imaging time may become long, or an examination may not be terminated in the worst case.
Therefore, as shown in, e.g., FIG. 4, a method of using a belt-like fixture which is a so-called abdominal belt 500 to fix an abdominal is used. This abdominal belt 500 enables obtaining a respiratory motion suppressing effect to some extent.
However, even if the abdominal belt 500 is used to fix the abdominal, the respiratory motion cannot be completely suppressed, and the respiratory level may fluctuate to prolong an examination time in long-time imaging. Furthermore, when fixing strength of the abdominal belt 500 is increased to reduce the respiratory motion, a burden on a subject may be enlarged. When the examination is prolonged, the subject may start moving because of discomfort caused by fixing. Moreover, when the subject has a large body, even the abdominal belt cannot be used.
On the other hand, there is a multi breath holding method of repeating breath holding rather than voluntary breathing for a plurality of times to image three-dimensional data.
As shown in FIG. 5, in the multi breath holding method, collection of data concerning one slab S1 including an entire heart is intermittently performed in synchronization with repetitive breath holding performed by a subject. In addition, there is a method which additionally uses RMC in the multi breath holding method and in which collection of data is performed only when a monitor signal is within an allowable range. However, it is hard for the subject to correctly understand his or her respiratory level. Even if the subject believes that he/she is uniformly holding breath, the respiratory level fluctuates in breath holding states. Therefore, when RMC is additionally used, a monitor signal may not fall within an allowable range even though the subject is holding breath, as shown in FIG. 6. In such a case, data is not collected even though the subject is holding breath, which imposes a load on the subject. It should be noted that the inadequate breath holding state lengthens a period where data cannot be collected, and an efficiency for data collection may be lowered, resulting in a long examination time. Additionally, when an imaging time is long, it is often the case that the subject gets tired of having to repeatedly hold his or her breath, the respiratory level in the breath holding state fluctuates further, and an examination cannot be terminated in the worst case. In the multi breath holding state that does not additionally use RMC, the data on a number of slabs is acquired in the state of different breath holding positions. As a result, reconstructed images may be discontinuous at the boundary between the slabs.
On the other hand, as shown in FIG. 7, there is considered a multi slab method of dividing a region including an entire heart into a plurality of slabs S1 to S4 and individually collecting data from each of slabs S1 to S4. To this case as well, either the simple multi breath holding method or the multi breath holding method that additionally uses RMC is applicable. FIG. 8 illustrates the case where the multi breath holding method that additionally uses RMC is applied, and an allowable range is changed in accordance with each slab. Since in this case the allowable range differs depending upon the slabs, a fault is inevitably produced in each slab if the collected data is used for reconstruction without any correction. A similar fault is produced in the case where the simple multi breath holding method is applied. The multi slab method has the following problems. In the case of the multi breath holding method that additionally uses RMC, the breath holding positions vary, and the collection of data cannot be performed efficiently, as in the above. In the case of the simple multi breath holding method, the breath holding positions vary each time, and blurring of each slab inevitably occurs.
As explained above, according to the voluntary breathing method, a fluctuation in the respiratory level and the long-term variation of the respiratory level degrade an efficiency of data collection based on a navigator echo method.
Further, when a combination of the multi breath holding method and the single slab method is applied, blurring occurs due to each-time variation of the breath holding position.
Where the multi breath holding method and the multi slab method are applied in combination, the breath holding position varies each time data is collected from one imaging region. However, the allowable range changes in accordance therewith, data is collected from different positions. Therefore, there is an inconvenience that a registration error is produced in a finally obtained 3D image and discontinuity of data occurs in this 3D image. Thus, to reduce such discontinuity, the respective slabs must be positioned in, e.g., image processing. However, since data positions included in the respective slabs are different from each other during data collection, appropriate positioning is difficult.
It is to be noted that relevant technologies are known from, e.g., JP-A 2000-041970 (KOKAI), JP-A 2000-157507 (KOKAI), or JP-A 2004-057226.